Fuel economy vs speed

Discussion in 'Powerboats' started by rattleandbang, Jan 11, 2016.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Something has gone horribly wrong in the motor(cade). Check the compression for gawd's sake.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Mail order . . .

    I have a 56 year old SBC I run as 4,800 RPM regularly. It's had a few replacement carbs, manifolds, rings, etc. in it's life.
     
  3. 7228sedan
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    7228sedan Senior Member

    Rattle, what is your intention with the boat?
    Are you looking to putt along at displacement speeds? If so, that will be the most economical assuming you keep the hull speed lower than when the planing event begins. If you want to burn a little more fuel but go significantly faster? Plane her off and find the sweet spot. Usually that's the lowest RPMs where you can keep the boat on plane. Trim tabs can help here.
    It's always recommended that the boat is able to operate at the full spectrum of RPMs. If you are getting only 3300 out of the motor in it's current configuration, something is wrong. I run my boat at 3000-3200 on plane & burn 10 GPH. It's a 28 Ft, old, heavy fiberglass sport fisherman with a single 454. It gets really thirsty if I rev the motor up. However there are time for safety sake that you need that power. If your motor is not capable of planing that boat off and getting the hell out of Dodge, I wouldn't recommend many day cruises until you know what's wrong with it.
     
  4. rattleandbang
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    rattleandbang Junior Member

    Putting around is what I prefer to do. My cruising grounds are the BC Gulf Islands, and there's no reason to be in a hurry. I'm used to sailboats so I plan on going slow, watch tides and currents and the weather. In 15 years of cruising this coast, I've never had to outrun anything - well, once I was caught in hurricane force winds off Cape Scott, but we dealt with it. I've just now finished a week long cruise, and that's during the stormy season. Yesterday it registered almost 40 knot winds just 10 miles south of here, but I had no problem making a run from Ganges on Salt Spring Island to Portland Island doing 6 knots. It's really only due to curiosity that I would like to see what it's like on plane, but I really doubt I'd ever cruise like that.
     
  5. Rurudyne
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    Rurudyne Senior Member

    Just the same, check the bilges to see if there's a hoard of silver and gold bars weighing her down.

    Hey, the odds are probably better than the lottery! :p
     
  6. rattleandbang
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    rattleandbang Junior Member

    Its a wooden boat. The gold is in her timbers and finish :)
     
  7. 7228sedan
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    7228sedan Senior Member

    I think she's beautiful, great classic lines. If you have the time to run like that then God speed, and enjoy it. Personally I wish I had the time to run at 6 knots :).
     
  8. Barry
    Joined: Mar 2002
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    Barry Senior Member


    "Your engines are designed for two modes of operation, idle and WOT, but for the most part, over 90% of the time the engines the engines will be cranking out near maximum output"

    The OP's boat is a gas engine. Please explain how an engine is designed for only two rpm conditions.
    I doubt anyone runs their gas engines at or near WOT due to excessive high fuel burn rates.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Most do throttle back 10% - 15% to save fuel and "cruise", but the engine is designed for a constant load and speed application, just like a generator, prop aircraft, etc. 90% of the time, a full plane mode engine will be at WOT or just off a bit to cruise. It's not designed to be flexible in regard to load or RPM. If it was, the ignition and cooling system would be setup to accommodate this need.
     
  10. rattleandbang
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    rattleandbang Junior Member

    But how much is this in the design? These blocks are off the shelf automotive engines marinized are they not? My Crusader 220 is a Chevy 287 block. except for bolt on components that must meet CG regs and such for fire, how is the block itself any different than an automotive one, which isn't designed for steady state operation? My antique motorhome had a dodge 413 industrial, an engine which was designed for pumps and generators, and was very much a heavy duty block, very different from a 413 auto block, but is it the same for these marine applications?
     
  11. PAR
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    PAR Yacht Designer/Builder

    The marine 4.7 block is essentially the same, but the cam, the ignition, induction, fuel delivery and exhaust aren't, as well as other ancillary elements, like the starter and alternator, etc. The 413 wedge Chrysler RB blocks were available in two configurations. The automotive variant was lighter, though still had the raised deck design, while the HD version had a thick casting throughout, thicker deck, valley and web reinforcements, etc.

    This isn't all that uncommon with engine manufactures - to offer a bigger, heftier version of the same engine. Exterior dimensions remain the same, but key internal dimensions, clearances and reinforcements are changed to handle higher loads. A classic example is the venerable SBC which started out as a 265 CID, 2 bolt main with a 2 piece rear seal, but eventually was cast to accommodate as much as 427 CID (safely) and I've seen aftermarket castings, that will permit 480 CID version of the same external dimensions (4.25"x4.25" bore/stroke). From a technical view, you get more HP from a bore increase, but you get more torque from stroke, which you can trade for HP with cam and induction dynamics.

    These same dynamics are used in industrial/continuous duty applications, such as boat engines. Automotive engines aren't considered continuous duty cycle rated, while prop driven aircraft, pumps, generators and boat engines generally are. This is why you'll see a 5.7 Crusader marine engine rated at 260 HP, while the same automotive engine, will be rated at 100 HP more, comparatively. The block is the same, the bore and stroke remain the same, but to guarantee 260 continuous HP from the same casting, in a continuous duty application, typically the power ratings are considerably lower, because of the different dynamics between the types. For example, you can take the same marine 350 and use a cam with higher lift, more duration and overlap and you'll quickly gain HP, but it's duty rating as a marine engine will drop considerably. Again, it the dynamics thing, such as the shortened valve overlap on the cam, making it possible to suck up water from the manifold, which is an engine killer on a boat, but not a concern on a car. There are many other variables to consider and though many parts are similar, if not the same as automotive applications, it's the assembled package that's the mitigating factor, which isn't necessarily easy to see, without some understanding of the differences.

    Yep, they're marinized automotive castings, but they're a lot more than a starter, alternator, exhaust manifolds and spark arrester on the carb.
     
  12. sdowney717
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    sdowney717 Senior Member

    How are you determining the 4GC air valve is not opening? Looking inside carb with engine in gear at WOT?

    The 4GC has a CFM air valve secondary on the inside, functions just like the quadrajet secondary air valve visible on top.
    If the engine is not able to flow the CFM of air, the valve of course will not open.
    Maybe nothing is wrong with 4GC carb and a quadrajet will not improve WOT.
    The engine lacks power, the carb is telling you the engine can not pump up the airflow CFM sufficient to open the air valve all the way.

    4GC just like Quadrajet did come in several size CFM ranges to fit various engine cubic inches. Maybe yours was replaced with a bigger 4GC?
    I had a large 4GC on a 401 Wildcat Buick engine.

    What is the Crusader 220, a 305 cubic inch v8? That is a smallish motor.
     
  13. PAR
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    PAR Yacht Designer/Builder

    The Crusader 220 HP is a 305 CID SBC and not the biggest available, but not the smallest either. It's the entry level first gen SBC from Crusader. The 350 is 260 HP.

    I agree this engine just needs to be checked out. There are several things that could be happening, but when there are known carb issues, this would be the first place to look once the basic tune specs are checked.

    Knowing Quads is import too. The secondaries opening amount is set by a set screw under the air horn, which winds this spring tension. A mis-aligned choke pull off can also affect secondary opening rates. These carbs often suffer from a warped air horn too. Other issues typical of these puppies are leaking well plugs, well feed tubes falling out of the air horn, broken accelerator pump springs, etc.

    These are easy to tune and reliable carbs, but they're different than a Holley or other carb you might be familiar with.
     
  14. rattleandbang
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    rattleandbang Junior Member

    What he said. Fascinating stuff. Still, it seems crazy roaring a V8 at those RPMs. I've only known them in road applications and due to their power and torque they're geared low, usually below 2000 at hwy speeds. I can't imagine the fuel flow of a V8 at that RPM with 4 open
    barrels.
     

  15. rattleandbang
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    rattleandbang Junior Member

    This Crusader is a 287 - they didn't have a 305 yet when this boat was built. And the carb is in very poor shape. I checked it with WOT and the secondary barely cracked. I agree the engine should have a tune up. It has new plugs but points and dwell should be checked. New cap and leads.

    Lining the floats with epoxy has worked for now, but no doubt there's corrosion elsewhere effecting things. The accelerator pump is toast, for example, not that I need it. It definitely needs replacing.

    I calculated yesterday that I was getting around 3 nmpg at 6.5 knots. An appalling rate, but I wonder if that's comparatively good or bad for one of these boats?
     
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